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Charting the molecular diversity on human skin

New anatomical maps are shining a light on the skin’s microbial and chemical diversity. James Mitchell Crow examines where all these chemicals come from.

The surface of our skin is a chemical wonderland. That’s what Pieter Dorrestein at the University of California, San Diego and colleagues found when they swabbed the bodies of two volunteers to map their skin’s microbial and chemical diversity. The results are represented here via these colourful dot maps – ranging from low diversity (blue) to high (red). The study was published in the Proceedings of the National Academy of Sciences.

Areas of high molecular diversity (red dots, left hand images) and high microbial diversity (red dots, right hand images) didn’t always align, suggesting microbes are not the main source of molecules on our skin. – Theodore Alexandrov

So where do all these chemicals come from? Skin cells and microbes make them and many come from the products we smear on ourselves. But chemical reactions that take place on the skin also create new compounds. More than 80% of the skin chemicals did not match any known compounds. The list of known compounds was dominated by ingredients in the volunteers’ hygiene and cosmetic products – especially around the head and face – even though the volunteers had not applied those products, or showered, for three days before the skin swabs were taken.

Molecules on the head and face are dominated by those we apply in soap, shampoo, sunscreen and cosmetics – lingering even days after being applied. – Theodore Alexandrov

While the foot had low microbial diversity, it was one of the areas of skin with the highest molecular diversity. This low correlation held for most of the body. But the team did find certain specific microbes and molecules that shared the same spots on the skin surface. Propionibacterium, which has been implicated in acne, was typically found in the presence of lipid molecules, which were in turn found in oilier areas of the skin: the head, face, hands, chest and back.

The foot is an area of low microbial diversity (left) but high molecular diversity (right) – Theodore Alexandrov

Using their mapping technique, Dorrestein and his team plan to start identifying the key molecules we need to maintain a healthy balance of microbes on the skin, and to study how this relationship can go awry to cause disease. They also plan to test how the ingredients in soaps, shampoos and cosmetics can alter the skin’s natural ecosystem.

The hands, unsurprisingly, are one of the more chemically diverse sites on the body. – Theodore Alexandrov